1. Field of the Invention:
The present invention relates to an ignition apparatus for thyristors and more particularly to an ignition apparatus for a plurality of thyristors connected in series and/or in parallel.
2. Description of the Prior Art:
Rectangular wave ignition apparatuses have been used in the past when a gate current having a fast rise time and a desirably controlled time interval is required for triggering a plurality of thyristors connected in series and/or in parallel. These rectangular wave AC generators generate a rectangular wave AC output for a predetermined period depending upon a control signal transmitted from a controlling device employing a rectangular wave AC generator, a controlling device for controlling the output period of the rectangular wave AC generator, a gate transformer and a full wave rectifying device. The rectangular wave AC output is transmitted through the gate transformer, rectified by a single phase full wave rectifying device connected to the secondary side of the gate transformer, and the resulting current is fed to the gate of one or more non-conducting thyristors.
In accordance with these rectangular wave type ingition apparatuses, the leakage inductance of the gate transformer may be decreased, the rise time of the gate current increased and the duration of the gate current interval lengthened by selecting a high output frequency for the rectangular wave AC generator. However, when a plurality of thyristors are connected in series and/or in parallel, it is not enough to feed them all simultaneously a gate current having only a high rise time because ignition characteristics of the thyristors are not uniform, and as a result an overvoltage may be applied to thyristors in the series connection of thyristors which do not immediately ignite. On the other hand, an overcurrent may also be applied initially to the first ignited thyristors in the parallel connection of thyristors. In both cases, thyristors may be damaged or destroyed, disadvantageously.
In the conventional rectangular wave AC generator, transistors or gate controlled thyristors having a relatively small current capacity are used as semiconductor elements. Accordingly, a large ignition capacity for igniting a plurality of thyristors by using only one ignition apparatus is not practical with previously known apparatuses. Thus, in the past a separate rectangular wave AC generator is disadvantageously required for each group of thyristors having the same ignition period. A need therefore exists for a novel apparatus having none of the disadvantages of the conventional apparatus.